The present invention relates generally to the control of metal producing, electrolytic cells, and particularly to a circuit arrangement providing control of the bath level within the cell without using measurements of the electrical resistance of the bath.
If molten aluminum is reduced electrolytically from a feed material such as aluminum chloride, for example, in a salt bath contained within a cell, the efficiency of the aluminum producing process within the cell depends upon maintaining a concentration of the aluminum chloride in the salt bath within appropriate, predetermined, percentage limits. This is accomplished by controlling the rate at which aluminum chloride is fed to the cell from a bin or hopper as aluminum metal and chlorine gas is produced within the cell. In the present invention such control is effected and cell efficiency maximized by measuring certain process variables, presently to be explained, and calculating a bath level based upon these variables that will automatically maintain the proper percent concentration of aluminum chloride (or other feed material) in the bath as reduction takes place, i.e., as aluminum and chlorine gas, for example, is produced and when the metal is removed from the cell.
Heretofore, however, control of the feeding of alumina or an aluminum halide to the salt bath of electrolytic cells employed in the production of aluminum has usually been effected by making contemporary measurements of the electrical resistance of the bath, as determined by the voltage existing across and the current flowing through the cell. Examples of such control procedures and apparatus are shown and described in such U.S. Pat. Nos. as Dirth et al 3,573,179; Shiver 3,622,475; Goodnow et al 3,812,024 and Haupin 3,847,761. Though the use of cell resistance as the controlling mechanism for the ordering of feed material to the cell has been a successful improvement over prior means and methods, the electrical resistance of a cell is affected by certain cell variables and conditions that make a different control mechanism desirable.
For example, a change in the temperature of the salt bath will change the electrical resistance of the bath. In addition, a layer of gas bubbles on an electrode of the cell can affect the resistance of a cell. Further, the shorting of a compartment of a cell changes the overall resistance of the cell. It can be appreciated that such conditions have nothing to do with the amount of feed material required for proper and efficient cell operation. Hence, the use of resistance measurements as a means to control the feed of material to the cell is not always reliable.
Further, as explained in the above Haupin patent, the standard curve for cell resistance versus concentration of an aluminum halide such as aluminum chloride (AlCl.sub.3) has a knee or inflection that can be misleading in regard to the need of further additions of AlCl.sub.3 to the cell, i.e. if the resistance reading is on the "wrong" side of knee, additional amounts of AlCl.sub.3 might be ordered when actually the cell is already oversupplied with AlCl.sub.3.